When would a contract be 'binding'?

I.e. If a house was on fire and a man trapped on the 2nd floor of the burning house shouted out that he would give all his money to the person who hands him a ladder so he can get down, does he HAVE to give that person who saved his life all his money?

and how come pre-nutial agreements are not binding?

I think, although I may be incorrect, that this would not constitute a binding contract. Perhaps a citizen will be under a general duty to help his fellow citizens and thus he will not be acting beyond these ordinary obligations and so is not providing consideration?

There is definitely no intention by the man to create legal relations. At that point in time, he would have said anything in order to get another person to give him a ladder. However, that having said, it must aslo be noted that if we were to say there was no contract, then we promote the idea that it is acceptable to make false promises, which I dont think the legal system should be potraying. But hey!, common law is the common law, its not always "just". I doubt a judge would say that it was a binding contract for lack of intention to create legal relations.